1. Field of the Invention
The present invention relates to a speaker diaphragm, and more particularly to a speaker diaphragm to which a voice coil bobbin can be attached with an improved bonding strength.
2. Description of the Related Art
FIG. 5 is a cross-sectional view showing a conventional speaker 500. The speaker 500 includes a bobbin 502, and a diaphragm 501 bonded to the outer surface of the bobbin 502. The outer surface of the bobbin 502 has a plain cylindrical shape (curved surface), and the adhesion between the bobbin 502 and the diaphragm 501 is dependent solely on the bonding strength of the adhesive used therebetween. Therefore, the vibration of the bobbin 502 cannot accurately be transmitted to the diaphragm 501 (some transmission loss occurs). The diaphragm 501 may come off the bobbin 502 due to deformation of the diaphragm 501 and/or an insufficient bonding strength. The vibration of the bobbin 502 produced according to an input signal is transmitted to the diaphragm 501 through a complicated path as indicated by arrows X1 to X4 in FIG. 5. Therefore, the vibration is reflected at the connecting portion between a dust cap 506 and the diaphragm 501, for example, thus increasing the transmission loss. A free end 502a of the bobbin 502 can act as an antinode of vibration and significantly bend and vibrate in the radial direction (the Y direction), resulting in unnecessary sound coming from the bobbin 502, thus deteriorating the S/N ratio.
The speaker of Patent Document 1 (Japanese Laid-Open Patent Publication No. 2002-125290) addresses the problem. FIG. 6 is a cross-sectional view showing an important part of a speaker 601 disclosed in Patent Document 1. In the speaker 601, a connecting portion 607 between a dome-shaped portion 605 and an edge portion 611 includes a bobbin-receiving groove 610 for receiving one end of a bobbin 602, which is bonded to the bobbin-receiving groove 610. The speaker 601 provides an improved bonding strength between a diaphragm 605 and the bobbin 602. However, the bobbin-receiving groove 610 is provided by forming a plurality of bends A1 to A7. The bends A1 to A7 significantly complicate the path through which the vibration of the bobbin 602 is transmitted to the dome-shaped portion 605 and the edge portion 611 via a horizontal portion 607a of the connecting portion 607. Thus, the vibration of the bobbin 602 is reflected at the bends A1 to A7, thus causing a transmission loss of vibration. Since the dome-shaped portion 605, the edge portion 611 and the connecting portion 607 are molded using the same material, the vibration of the bobbin 602 in the radial direction is transmitted to the dome-shaped portion 605 and the edge portion 611 via the connecting portion 607, resulting in distortion.
Patent Document 2 (Japanese Laid-Open Utility Model Publication No. 6-34397) discloses a speaker in which a reinforcement ring is provided at the distal end of the voice coil bobbin for increasing the bonding strength between the bobbin and the diaphragm. The outer periphery portion of the reinforcement ring forms an inclined surface that conforms with the surface of the diaphragm. The reinforcement ring is bonded to the bobbin and the diaphragm, and can improve the bonding strength between the bobbin and the diaphragm. However, the provision of the reinforcement ring adds to the number of components and the number of production steps. Moreover, the reinforcement ring has an adverse influence on the sound quality.